Various machine tools, such as lathes, incorporate means for chucking and rotating a workpiece to be machined. In many such machines, the chucking mechanism is power operated, through an hydraulic actuating means. In a typical rotary machine tool, the work holding chuck is mounted for controlled rotation relative to a cutting tool and includes a plurality (frequency three) of radially movable gripping jaws. These gripping jaws are adapted to be controllably advanced into pressure gripping contact with the workpiece, which is normally a body of revolution.
During rotation of the chucking device, centrifugal force acts outwardly against the radially movable gripping elements, tending somewhat to counteract and reduce the radially inward gripping force applied through the hydraulic actuating system. Such outward centrifugal forces become increasingly consequential with increasing chucking diameters and also, of course, with increasing speeds, effectively subtracting from the initial gripping force on the workpiece. It is possible, in some cases, to compensate for the effective centrifugal forces on the gripping elements by setting the initial gripping force to be larger than necessary, by an amount approximating the anticipated loss of gripping effect through centrifugal force. However, this is impracticable in many instances and impossible or undesirable in others. For example, where the workpiece is subject to crushing or distortion by excessive chucking force, it would be altogether inappropriate to attempt to overcome centrifugal force by overloading the chucking forces in the first instance.
The problem of loss of chucking force through centrifugal force has been recognized in the past. Early attempts to compensate for centrifugal force have involved the use of counterweight mechanisms. However, these mechanisms inherently suffer disadvantage of requiring extra weight and bulk. Particularly as the turning equipment becomes larger and is designed to operate at increasingly higher speeds, arrangements for mechanically counterweighing the gripping jaws become entirely impractical.
A somewhat more effective solution has been marketed heretofore by The Bullard Company, Bridgeport, Conn. This system has utilized a manually adjustable pressure regulator mechanism for establishing the initial force levels of the gripping elements upon the workpiece. During rotation of the chuck, its speed of rotation is sensed, and an electronic signal proportional to the speed of rotation operates through a servo valve system to incrementally increase the initial pressure setting as a function of the amount of centrifugal force introduced by such rotational speed. This system represented a substantial advance over mechanical couterweighting systems, in that the chuck and rotating parts did not require the massive and unwieldy counterweighting units characteristic of the mechanical devices.
In accordance with the present invention, a new and improved chuck force compensating system is provided, which utilizes hydraulic pressure compensation, somewhat in the manner of the earlier Bullard control described above, but which incorporates significant advantageous features providing superior performance and enabling the system to be fail-safe in operation. As will be appreciated, the need for chuck force compensation becomes increasingly acute with parts and chucking mechanisms of greater size and operated at higher speeds. Concomitantly, the consequences of a failure in the system become increasingly significant with higher speeds and weights, such that the need for fail-safe operation is parallel with the need for chuck force compensation.
In accordance with the present invention, a novel and improved power chucking system is provided, which incorporates, in addition to speed-sensing chuck force compensation means, a fail-safe system effective to shut down machine operation if the chucking force applied is in fact less than that which is scheduled for the operating conditions. To this end, the system of the invention incorporates a fully electronic system for chuck force control, in which the initial chucking force, as well as the compensation for rotational speed, is effected by the inputting of controlled electrical signals, such that the actual chuck force applied is a resultant of a so-called static signal, reflecting the initial chucking force, and a dynamic signal responsive to rotational speed. This inputting arrangement, in conjunction with a feedback system responsive to chuck force actually applied, provides for continuous self-monitoring of applied chuck force versus scheduled chuck force and bringing about stoppage of the machine if there is a consequential difference.
In accordance with another aspect of the invention, a redundant, parallel system is provided for creating a signal voltage comparable to the control voltage developed by the primary system. The secondary voltage is constantly compared to the primary signal voltage and, in response to any consequential difference, a shut down of the machine is effected. Thus, in the primary control system, there is a possibility, all be it relatively remote, that the primary control signal, by reason of malfunction in the signal generating and modifying circuitry, could experience a so-called decay fault, resulting in gradual reduction or loss of the control voltage. If this occurs sufficiently gradually, the servo system controlling the chuck gripping elements, and the feedback system monitoring the applied gripping force can respond, resulting in gradual loss of gripping force without indication of fault. In the system of the present invention, however, by providing a redundant signal, from a separate and independent source, it is possible to make a constant comparison of the primary control signal and the redundant signal. If at any time there is a consequential difference between the two, a fault signal is generated. To particular advantage, the redundant signal may utilize the output signal from the tach generator normally provided with the rotating equipment, while the primary signal is derived from an entirely independent sensing element.
In accordance with another apsect of the invention, provision is made for safe coast-down of the equipment in the event of total power failure, which would of course result in the loss of control signal to the servo system controlling the power operated chucking elements. The system includes a gas-hydraulic accumulator, which becomes charged during normal operation of the hydraulic system feeding the power operated gripping elements. Normal flow of hydraulic fluid to the chuck actuator is through a servo-operated pressure control valve, which operates in response to the speed-responsive control system described above. A bypass circuit is provided around the servo-operated pressure control valve, through a normally open solenoid actuated valve. During all power-on conditions, the solenoid valve is actuated to a closed condition. However, upon any kind of catastrophic power loss, the bypass valve reverts to an open position, bypassing the servo pressure control valve and connecting the gripping device directly to the hydraulic accumulator, which maintains pressure until the machine comes to a safe stop.
For a more complete understanding of the above and other features and advantages of the invention, reference should be made to the following detailed description of a preferred embodiment and to the accompanying drawing.